Surfactant systems have received much attention recently as a means for increasing the recovery of oil from a subterranean reservoir. Typically these systems employ a petroleum sulfonate as the surfactant, and an alcohol as a cosurfactant or cosolvent. Thus, when the oil and brine (water plus electrolyte) are also considered, these oil recovery systems are seen to contain at least five components.
Because of the high cost of surfactant systems, it is important that any such system be optimized to provide the greatest oil recovery at the lowest cost. Unfortunately, this optimization is hindered by, at least, the following three factors:
1. The large number of components and the correspondingly large number of possible compositions which must be evaluated. PA1 2. Interactions between components which make interpolation of behavior difficult. PA1 3. The relative difficulty of performing displacement tests in porous media.
For example, if samples with ten concentrations for each of five variables were prepared, there would be on the order of 10.sup.4 samples and an optimum would still not be defined. Obviously, it is not feasible to study this many samples, but rather systematic steps must be developed for predicting the behavior of samples with compositions intermediate between the samples which have been studied. Interactions among the various components make such interpolation difficult. For example, if the "optimal salinity" is found while holding the other variables fixed at some value, that salinity will not be optimal when the remaining variables are fixed at another value. Thus it is not feasible to sequentially optimize with respect to each single variable, but rather the ultimate optimum is a function of all variables and must be treated as such.
Compounding the difficulty is the need to evaluate various surfactant systems by oil displacement tests in porous media. These tests are slow and expensive and a substitute test (at least for screening purposes) is highly desirable. For the preferred embodiment herein, the criterion for optimization is taken as a minimization of interfacial tension. Interfacial tension, as related to the capillary number, has been shown to be a key factor in the displacement of residual oil. Furthermore, interfacial tensions can be measured by the spinning drop technique with relative ease. In a three-phase sample there exist three interfacial tensions and since the largest of these is the one which controls the displacement, the optimization scheme must minimize this largest tension by adjusting the composition of the system. While the minimization of interfacial tension may or may not coincide with optional oil recovery, nevertheless low interfacial tension is a necessary condition for achieving high displacement efficiency (under low pressure gradients).